Information is stored and manipulated in data processing systems in several binary forms. Among the most common are straight binary (a series of ones and zeros representing, at each digit, whether or not the power of two at that position is a constituent of the represented number) and Binary-Coded-Decimal BCD (one or more groups of four binary digits, each group representing a decimal digit, the legal groups thus extending from 0000 representing 0.sub.10 to 1001 representing 9.sub.10). It is often necessary to convert between binary and BCD in preparation for conducting various operations or in concluding an operation, and numerous algorithms and procedures for effecting the conversions per se are well known in the art.
As central processing units of information processing systems have increased in power and speed, it has been necessary and useful to correspondingly increase their level of integration (and hence dramatically reduce their size), and virtually entire central processing units have been implemented on a single VLSI chip. However, the most powerful mainframe central processing units, because of their complexity, typically employ several VLSI chips which may be situated on a single printed circuit board.
As previously noted, instructions for effecting conversion between binary and BCD numbers has been a feature of mainframe computers for many years, and it might be thought that an effective implementation of such conversion features in an earlier, central processing unit employing less dense integration might simply be copied to a VLSI central processing unit. It has been found, however, that such is not necessarily the case because, in a single printed circuit board central processing unit, the division of the computing burden among the several VLSI chips will often not correspond with the distribution of the computing burden among the several logic blocks of an earlier, less densely integrated central processing unit. Therefore, it may become necessary to redevelop and transmute even such apparently straightforward computing operations as data conversion in order to implement these operations into single printed wiring board central processing units, and it is to the realization of these conversion operations in the VLSI environment that the present invention is directed.